In tractor transmissions such as that disclosed in my above-referenced application, the clutch plate engagement pressure, hereinafter referred to as the clutch pressure, applied during shifting from one gear to the next is increased at a rate which does not take into account the load on the tractor drawbar. The rate of increase of clutch pressure is ideally set for optimum operation at full load conditions but for light or no-load conditions this causes a "jerk" or sudden acceleration. Thus, a trade-off is made. The rate of increase of clutch pressure is made lower than that required for optimum operation at full load in order to obtain a smoother acceleration at light loads. While this trade-off has been satisfactory for most shift conditions, a suitable trade-off has not been found for certain conditions such as the shift from 9th to 10th gear in the transmission disclosed in my prior application.
It is possible to tailor the shift strategy for varying load conditions by modifying the rate of clutch pressure increase in accordance with the load, but the system shown in my copending application has no load sensor. It does, however, have a speed sensor for sensing ground speed by sensing the rate of rotation of the transmission output shaft and in accordance with the present invention this speed sensor may be used to develop signals indicative of the load being pulled by the tractor.
Use of a speed sensor to obtain an indication of load is enhanced because the transmission output shaft and tractor driveline wind up or are twisted as the transmission applies torque to the shaft, and as the transmission ceases driving the shaft, the shaft and driveline unwind. The degree of winding and unwinding is dependent on the tractor load and causes an apparent ground speed variation as sensed by the ground speed sensor. This variation may thus be used to adapt the rate of increase of clutch pressure according to the load so that optimum smooth shifting is obtained for all load conditions.